Grid connected power converters are devices in which semiconductor switch components are used for producing alternating voltage for supplying power to the grid. Such converters are used, for example, in connection with solar power and wind power, and generally in power generating system in which a rotating generator is not synchronized with the grid to which power is to be fed.
An example of a grid connected converter is a three-phase voltage source inverter the output of which is connected to the grid. The gridconnected converter is typically referred to as a grid side converter (GSC). The grid side converter is connected to a DC link and power is fed to the grid side converter through the DC link from a machine side converter (MSC). The machine side converter is connected to a rotating generator which is rotated by an energy source, such as wind or water.
In connection with solar power generation, the produced power is DC, and therefore machine side converter is not required. Instead, the DC voltage produced with the solar generator is modified to a suitable level such that the grid side converter can feed the produced power to the network.
Typical full power wind converter systems comprises a wind turbine controller (WTC) which provides an external AC voltage reference Uref1 and active power reference Pref to a power converter system. The wind turbine controller is an upper level controller which controls the overall operation of the converter system and related functions pertinent to the operation of the wind turbine such as but not limited to blade pitch control, communications with a higher level wind park controller. The power converter system has a grid side converter connected to the power grid by means of a wind turbine transformer and the collector system of the wind power park, and a machine side converter controlling the generator and a DC link connecting the two converters.
In the grid side converter an AC voltage controller regulates the grid side AC voltage and produces a reference value Ir_ref for reactive current. Further a DC-link controller regulates the DC voltage of the DC-link to a reference value and produces an active current reference Ia_ref.
Further, a typical implementation of a grid side converter includes a current limiter which enforces the currents to be within a certain limit. The current limiter can operate such that the current references Ir_ref and Ia_ref are kept under a limit Ilim according to Ilim≥√{square root over (Ia_ref2+Ir_ref2)}. The limit Ilim is the maximum continuous current limit of the grid side converter. The current limiter operates in active current priority and produces limited current references Ia_ref_lim and Ir_ref_lim. With the active current priority it is referred to the operation principle according to which the reactive current reference Ir_ref_lim is appropriately lowered in case the said limit is reached or exceeded.
The grid side converter further comprises controllers for regulating active Ia and reactive Ir current injections towards the grid in response to their limited references Ia_ref_lim and Ir_ref_lim.
In a typical full power wind converter system the AC-voltage controller of the grid side converter follows the AC voltage reference Uref1 which is received from the wind turbine controller. The active power reference Pref which is received from the wind turbine controller is converted into a corresponding generator torque reference for the machine side converter which controls the wind turbine generator correspondingly. The power produced by the generator passes through the machine side converter and is injected into the DC-link. The DC-link voltage is regulated by the DC link controller of the grid side converter. The GSC of wind turbines are connected in many practical cases to very weak grids, that is so say, grids with such a high network impedance resulting in a low Short Circuit Ratio (SCR) at the wind power park grid connection point in the order of 1 . . . 2.
In case the above described converter systems are connected to very weak grids and are operating at or near nominal active power, they are frequently prevented operating in a stable manner within their nominal AC voltage operating band due to their current limit Ilim. This unstable operation is also referred to as ‘voltage collapse’ and is highly undesirable as it prevents normal power production.
This phenomenon is due to physical line transfer limits on realizable active power, reactive power and voltage (P,Q,U) points. These physical limits necessitate that in order to transfer a certain amount of active power P towards the grid a certain minimum voltage U must be maintained which, in turn, will lead to sufficient reactive power in order to ensure voltage stability. As a consequence, for a given Pref issued by the WTC, there exists a minimum AC voltage reference Uref1 which must be provided by the WTC to the converter system, in order to prevent a voltage collapse at the GSC terminals.
The chain of events that lead to the voltage collapse at the current limit Ilim is the following. When the AC voltage reference Uref1 is lowered, more active current is needed to create the same amount of active power according to Pref. Eventually when reaching the current limit Ilim, the required reactive current reserve √{square root over (Ilim2−Ia_ref_lim2)} is used up, since the active current requirement increases faster than the reactive current requirement drops as a function of the converter AC voltage. Active current has priority, when the current limit Ilim is reached. This causes a reduction in reactive current, which again reduces the AC voltage at the GSC terminals, which again will result in an increased active current requirement. This chain of events will end up in the collapse of the converter AC voltage outside the nominal voltage operating band, driving the GSC into fault-ridethrough mode or make it trip.
This minimum AC voltage reference Uref1 to be provided by the WTC increases with the grid X/R and decreasing short circuit ratio (SCR). For extreme weak grids with SCR=1 and moderate to high X/R the GSC must operate at elevated (>1 pu) AC voltage references in order to prevent voltage collapse.
The realizable (P,Q,U) points could be identified a priori by static load flow analysis (LFA) for a known grid scenario and stored in form of a look-up table within the WTC in order for the WTC to be able to issue appropriate active power and voltage references to the converter system Preventing voltage instability
However, the actual variables, as for example wind farm SCR and loads near the connection point, are either unknown to the WTC or hard to estimate reliably. So in practice, the wind park controller and consequently the WTC will need to reserve a considerable ‘safety margin’ on the converter AC voltage reference Uref1 and/or Pref in order to minimize the risk of entering voltage collapse. However, reducing Pref (i.e. power curtailment) is undesirable as this will reduce the revenue of the wind farm and increasing Uref1 to an unnecessarily high value will limit the wind converter and thus wind park Q-capability which is undesirable from the perspective of the grid operator.
It would thus be desirable to operate the wind converter system in such a way that the active power reference is realized and the AC voltage reference is either realized or only minimally increased in order to facilitate stable operation of the converter.